Walmart offers the Intex Twin 8.75" Classic Downy Inflatable Airbed Mattress for $7.97. Opt for in-store pickup to dodge the $5.99 shipping fee. That's tied with our Black Friday weekend mention and at least $8 less than what third-party sellers charge via eBay and Amazon. It measures 39" x 75" x 8.75".
None of these moves requires a big investment, and they all can save you money.